A light source which is fabricated integrating a DFB (distributed feedback) laser with an EA (electro-absorption) modulator on a substrate has a very small wavelength variation(chirping) during the modulation operation. Therefore, it is important as a light source for more than several tens to hundreds kilometer long-distance optical communication at more than 2.5 to 10 Gb/s. When the DFB laser and the EA modulator are monolithically integrated, it is required that regions with different bandgap energies are formed on the substrate since they have different operation wavelengths(bandgap energies).
H. Soda et al., "HIGH-POWER AND HIGH-SPEED SEMI-INSULATING BH STRUCTURE MONOLITHIC ELECTROABSORPTION MODULATOR/DFB LASER LIGHT SOURCE", ELECTRONICS LETTERS, Vol. 26, No. 1, pp. 9-10(1990) [First Prior Art] discloses a semiconductor optical device where a DFB laser and an EA modulator are monolithically integrated as shown in FIG. 1. In fabrication, a grating 201a is partially formed on a n-InP substrate 201, then an optical guiding layer 103, an etching stopper layer 212 and a laser active layer 204a, which are a laser layer structure, are grown on the entire surface of the substrate, then only the laser active layer 204a corresponding to a modulator region is selectively etched and an optical absorption layer 204b is formed there by burying regrowth, and a p-InP cladding layer 206 and a cap layer 207 are finally grown, whereby a butt-joint structure is obtained. In this structure, a relatively high optical coupling efficiency more than 80% between the laser and the modulator is obtained. However, it is difficult to control the etching and the buried-regrowth to reproducibly obtain a good structure.
M. Aoki et al., ELECTRONICS LETTERS, Vol.27, No.23, pp.2138-2140(1991) and IEICE, Autumn Meeting C-96, preparatory papers No.4-176(1993) [Second Prior Art] disclose a DFB laser/optical modulator jointed structure where the width of a growth-blocking mask can be varied by selective MOVPE to control bandgap energies of waveguide, thereby achieving an optical coupling efficiency of about 100%.
In fabrication, as shown in FIG. 2A, a pair of growth-blocking masks 302 with a mask width of several tens to hundreds .mu.m are formed only on the laser region of a n-InP substrate 301 while having a spacing of several tens .mu.m. Then, an optical guiding layer 303, an active layer 304 and p-InP cladding layer 307 are, as shown in FIG. 2A, grown by selective MOVPE. Then, as shown in FIG. 2C, an optical waveguide with a width of 1.5 to 2.0 Mm is formed by mesa-etching both the laser region and the modulator region, and then a Fe-doped InP layer 313 as a high-resistance layer is grown burying on both sides of the optical waveguide.
However, in the above fabrication method, there are problems that it needs the semiconductor mesa-etching process to form the optical waveguide, where width, height etc. of the mesa have to be severely controlled, and that the product yield is therefore reduced.
On the other hand, T. Kato et al., ELECTRONICS LETTERS, Vol.28, No.2, pp.153-154(1992) and IEICE, Spring Meeting C-226, preparatory papers No.4-223(1994) [Third Prior Art] disclose a DFB laser/EA modulator integrated light source where an optical waveguide is fabricated without etching a semiconductor layer while having good controllability and reproducibility.
In fabrication, as shown in FIG. 3A, a pair of SiO.sub.2 stripe masks 20 with a spacing of 1.5 to 2.0 .mu.m are formed in the [011] direction on a n-InP substrate 21. Then, as shown in FIG. 3B, an optical guiding layer 22, an active layer 23 and a p-InP cladding layer 24 are formed on a region sandwiched between the SiO.sub.2 masks by selective MOVPE. At this time, on the side facet of the optical waveguide composed of these layers, a (111)B crystalline surface is automatically formed. Therefore, a mesa-stripe structure with a high uniformity can be produced.
Then, as shown in FIG. 3C, the SiO.sub.2 masks 20 as a frame for the mesa stripe are widened, thereafter a p InP layer 25 and a p.sup.+ -InGaAs cap layer 26 are epitaxially grown by selective MOVPE. Finally, forming electrodes, a laser structure as shown in FIG. 3D is obtained.
As described above, the optical waveguide can be directly formed by using selective MOVPE, where the mesa-etching of the semiconductor layer is not necessary. Thus, the DFB laser/EA modulator integrated light source with good controllability and reproducibility as well as a high product yield can be fabricated.
However, in the above fabrication method, a current blocking structure cannot be provided. Thus, there are problems that a threshold value for laser oscillation cannot be reduced and that it is difficult to operate using a high light output power.
As a solution of the problems of the third prior art, Y. Sakata et al., TECHNICAL REPORT OF IEICE, LQE95-88, PP.39-44(1995) [Fourth Prior Art] discloses a 1.3 .mu.m strained MQW BH-LDs with a current blocking structure.
In fabrication, a pair of SiO.sub.2 stripe masks 2-1, 2-2 with an opening width of 1.5 .mu.m are, as shown in FIG. 4A, are formed in the [011] direction on a n-InP substrate 1, then an optical waveguide composed of an optical guiding layer 3-1, an active layer 3-2 and a cladding layer 3-3 is directly formed on a region sandwiched between the SiO.sub.2 masks 2-1, 2-2 by selective MOVPE, like the third prior art. Then, as shown in FIG. 4B, a SiO.sub.2 mask 4 is formed on the optical waveguide. 15 using this as a growth blocking mask, the current blocking structure composed of a p-InP layer 5 and a n-InP layer 6 is buried(FIG. 4C). Then, after removing the SiO.sub.2 mask 4, a p-InP cladding layer 8 and a P.sup.+ -In.sub.y Ga.sub.1-y As cap layer 9 are formed as shown in FIG. 4D. Finally, mesa-etching to form a groove for separating the device and forming 20 electrodes, a semiconductor laser as shown in FIG. 5 is obtained. Thus, a semiconductor laser, which can be operated at a low threshold and a high efficiency, can be produced with good uniformity and controllability.
However, in the above fabrication method, it is difficult to form the narrow mesa structure with a width less than about 10 .mu.m while having good controllability and a small dispersion among produced devices since the groove for separating the device needs to be formed by mesa-etching. Also, due to the steep side wall of the narrow mesa structure itself, the breaking of electrodes may occur. Thus, due to the difficulty in the formation of the narrow mesa structure, particularly, when a laser/modulator integrated light source is fabricated, the device capacity of the modulator part may be increased and the high-speed operation at higher than 1 Gb/s may not be therefore achieved.